Magnetoresistance (MR) effects are utilized to detect variations within a magnetic field by converting into variations of resistance. Materials that exhibit MR effect also often exhibit spin-polarization ability. Therefore, they can form spin valves to transmit signals based on spin polarization. Recent advances in data storage technologies have been based upon the use of multilayer metallic thin films that exhibit giant magnetoresistance (GMR) (e.g., U.S. Pat. No. 5,977,017 (Golden et al.)). MR-based devices, e.g., magnetoresistors, magnetic heads to detect signals of a magnetic recorder, or probes of a spin-polarized scanning type tunneling microscope, have the advantage of high immunity to radiation damage. Thus, data storage is indestructible, even under adverse operating or storage conditions.
Oxides are known to have large MR properties. In a class of oxide materials called manganates (see below), such large MR has been termed colossal magnetoresistance (CMR). (Ramirez, J. Phys.: Condens. Matter 9:8171-8199 (1997)). CMR is also commonly used to refer to the large MR observed in similar oxides as manganates when cobalt is used to replace manganese, called cobatates.
In addition to the large MR effect, oxides are also known to exhibit electric-pulse induced resistance (EPIR) properties. (Liu et al., Appl. Phys. Lett. 76(19):2740-2752 (2000)). EPIR switching effects are utilized to permanently change resistance, until a reverse electrical pulse is applied. Liu et al. teach a method for switching the properties of perovskite materials used in thin film resistors by applying short electrical pulses to change (both reversibly and non-reversibly) the electrical, thermal, mechanical and magnetic properties of the material without damaging it (Liu et al., Appl. Phys. Lett. 76:2749-2751 (1999); U.S. Pat. No. 6,204,139). This permits the formation of memory devices and resistors in electronic circuits that can be varied in resistance. Materials exhibiting an EPIR effect are useful as non-volatile, rewritable memory cells to store information in microelectronic devices. For example, U.S. Pat. No. 6,473,332 (Ignatiev et al.) teaches an electrically operated, overwritable, multivalued, non-volatile resistive memory element that includes a two terminal non-volatile memory device using CMR oxide film material and a defined circuit topological configuration that takes advantage of the variable EPIR effect of the thin film material.
For the purpose of the present application, perovskite compositions refer to the formula ABO3, in which A represents a metal that may be drawn from alkali, alkali earth, rare earth, or other metal such as potassium, strontium, lanthanum, neodymium, cerium, yttrium, lead, bismuth or the like, and B represents a transition metal such as cobalt, iron, nickel, or the like. (Perovskite actually can have even broader compositions for metal substitution, as will become clear later in the following description.) Perovskite-type materials have long been known to be useful for the catalytic oxidation and reduction reactions associated with the control of automotive exhaust emissions (U.S. Pat. No. 4,107,163 (Donohue)), but the electrical conductivity characteristics of the material are more recently discovered, such as the CMR effect observed in manganites, when B represents manganese (Mn), and cobatates, when B represents cobalt (Co) (Ramirez, 1997). Kobayashi et al., also made various studies on other, non-Mn and non-Co, ordered perovskite oxide crystals and identified a MR phenomenon, permitting the development of a magnetoresistor that is an oxide crystal with an ordered double perovskite crystal structure (Kobayashi et al., Nature 395:677-680 (1998); U.S. Pat. No. 6,137,395). (See below on Sr(Fe0.5Mo0.5)O3 and Sr(Fe0.5Re0.5)O3).
As made clear from above, for the purpose of the present application, manganates refer to a known class of perovskite oxides in which B represents manganese (Mn). Many manganates have CMR properties. Consequently, upon application of a magnetic field, the electrical resistivity of the material drops drastically due to a field-induced switching of the crystal structure. However, manganate MR is limited to a certain temperature range in which the magnetic field can promote (or in some compounds induce) a phase transition. The resistance response to the magnetic field is often hysteretic, in that a different resistance is found at the same magnetic field depending on the history of the field, such as whether the field has been increasing or decreasing. In addition, there is significant 1/f noise associated with circuits comprising manganate MR materials. It is generally accepted that in perovskite compositions, manganese may be held in both the trivalent Mn3+ state and the tetravalent Mn4+ state, and that such mixed valency is an essential element for the phase transition and the resultant large MR properties of the compound. Very similar mixed valency also exists in cobaltates that exhibit CMR effect.
Some manganates and cobaltates exhibit both CMR and EPIR effects. Generally, most EPIR effects were observed in thin films, across which a large electrical field may be achieved using a relatively modest electrical voltage. For example, Liu et al. describes such effect in CMR (Pr0.7Ca0.3)MnO3 thin films. (Liu, 2000). In the patent of Liu et al. (U.S. Pat. No. 6,204,139), an additional cobaltate of a metal composition Gd0.7Ca0.3BaCO2 was described as having EPIR properties. According to Liu et al (U.S. Pat. No. 6,204,139 B1), other oxide materials of the perovskite-related families that exhibit EPIR effect include YBa2Cu3O7 which is also known to possess high Tc superconductivity.
Mixed valency also appears to be important for the EPIR effect. The manganate that is best known for having the EPIR effect, (Pr0.7Ca0.3)MnO3, contains 70% Mn3+ and 30% Mn4+. Both cobaltateand the high Tc superconductor described above are known to have mixed valency. However, at least in the case of manganates, a disadvantage of such mixed valency is that manganates are sensitive to surface conditions. As a result, Mn ions at a surface that is exposed to air, vacuum, or other atmospheric or liquid environment, or at the interface that is adjacent to another solid material, can have different degrees of mixed valency distinct from those in the interior, depending on the moisture and chemical nature of the environment. As a result of such sensitivity, the magnetic (MR) and electrical (EPIR) responses of the resistivity are sometimes difficult to control or predict. In addition, mixed valency is sensitive to radiation, laser illumination, and other non-magnetic and non-electrical stimuli, which could also affect the magnitude of the observed MR and EPIR effect.
Several other families of oxides are also known to exhibit large MR effects, or large EPIR effects, but not both. One family of oxides that have been known to have large MR properties is based on the composition Sr(Fe0.5Mo0.5)O3 mentioned previously, which also has a perovskite-type structure ABO3. In this case, however, B is comprised, in equal parts, of Fe and Mo, which are ordered. Thus Fe and Mo are located on alternating sites in a checkerboard type of arrangement. The MR capability of this material results from a tunneling effect across grain boundaries. Therefore, substantial magnetoresistance is lost when Sr(Fe0.5Mo0.5)O3 is formed into a single crystal or an epitaxial film. It is believed that a 1:1 ordered cation arrangement is important for its magnetoresistance properties. However, disadvantageously, Mo in the above compound has a valence state of Mo5+, as opposed to the more common Mo6+ state, therefore a careful preparation under a reducing atmosphere or vacuum is required to form such compounds. Another ordered perovskite, Sr(Fe0.5Re0.5)O3, reportedly exhibits similar MR properties, and suffers from the same limitations. These compounds are not known to exhibit EPIR effect.
By comparison, Beck et al., have claimed a large number of oxide families that reportedly exhibit EPIR effect (PCT application, PCT/IB00/0043)). However, the examples in the PCT application and publications by the same group of researchers at IBM Zurich reveal that only the following oxides demonstrated the claimed EPIR effect: doped Ba1-xSrxTiO3, when doped with p-type (chromium or manganese) or n-type (vanadium or niobium) dopants; chromium-doped SrZrO3, doped Ca2Nb2O7 and doped Ta2O5 with chromium or vanadium as dopants (see, Beck et al., Appl. Phys. Lett. 77(1):139-141 (2000); Watanabe et al., Appl. Phys. Lett. 78(23):3738-3740 (2001); Rossel et al., J. Appl. Phys. 90(6):2892-2898 (2001)). Earlier reports also indicated that Al2O3, Nb2O5, TiO2, Ta2O5 and NiO may exhibit memory behavior based on current-induced bistable resistance switching or voltage-controlled negative resistance phenomena (see references cited by Beck et al., 2000). None of the cited materials, however, exhibit a large MR effect.
Many ruthenate oxides, on the other hand, are known to be excellent conductors. For example, strontium ruthenate (SrRuO3), which is also a perovskite, is often used as a bottom electrode material in electronic devices (Eom et al., Science 258:1766-1769 (1992); Eom et al., Appl. Phys. Lett. 63:2570-2572 (1993); Tiwari et al., Appl. Phys. Lett. 64:634-636 (1994); Klein et al., J. Magn. Mater. 188:319-325 (1998); FIG. 1 of PCT/IB00/00043). SrRuO3 itself is metallic and ferromagnetic (Longo et al., J. Appl. Phys. 39:1327-1328 (1968)), which is unique among 4d transition metal oxides. It has a very small (0.5-4%) MR, restricted to a narrow temperature range near the ferromagnetic/paramagnetic transition temperature. When SrRuO3 electrodes were used in place of the Pt layer in Pb(Zr,Ti)O3-based ferroelectric memory devices, they alleviated the problem of polarization fatigue (Eom et al., 1993). It is, however, not known to have EPIR effect.
The ABO3 perovskite structure of SrRuO3 also makes it compatible with other similar compounds, allowing its incorporation as an epitaxial thin film or buffer layer in heteroepitaxial device structures built on perovskite oxides (Tiwari et al., 1994). Further, substitution of Ru by other transition metal ions (Mn, Fe, Co, etc.) has been reported to create new magnetic properties, ranging from colossal magnetoresistance (e.g., in Sm, Ca(Ru,Mn)O3, with Ru in amount considerably less than Mn) (Raveau et al., J. Supercond. 14:217-229 (2001)) to spin glass behavior (e.g., in Sr(Fe0.5Ru0.5)O3) (Battle et al., J Solid State Chem. 78:281-293 (1989)). Related Ru-based compounds further display an exceptionally rich variety of electronic and magnetic properties, ranging from paramagnetic (e.g., in CaRuO3) to superconducting (e.g., in Sr2RuO4) while retaining metallicity (Longo et al., 1968; Maeno et al., Nature 372:532-534 (1994)). None of these Ru-based compounds or transition-metal substituted ruthnates are known to have EPIR effect.
Another ruthenate composition, TlSrRuO (Tl:Sr:Ru at a ratio of 1:2:1), which is believed to have a layered structure that is derived from perovskite, were reported to have magnetic transitions (e.g., U.S. Pat. No. 5,759,434 (Shimakawa et al.)). As a result, magnetoresistance in the vicinity of the transitions have been observed. This material is highly toxic, however, because of the presence of Tl. It is also not known to have EPIR effect.
A review of the ruthenate literature reveals that no EPIR effect has been reported, but considerable difficulties have been encountered in synthesizing these compounds. Consequently, highly uniform materials free of second phases or heterogeneous clusters are difficult to obtain using standard solid state reactions of mixed starting oxide powders. Often, very long calcination and/or laborious regrinding and remixing is needed, especially when two B-site cations are desired in the resulting compound (Battle et al., 1989; Kim et al., J. Solid State Chem. 114:174-183 (1995); He et al., Phys. Rev. B63:172403 (2001)). It is also known that even minor second phases and small heterogeneous clusters can have a considerable effect on the magnetic and electrical properties of these compounds (Kim et al., 1995; He et al., 2001).
Accordingly, there has, until the present invention, existed a need for commercially useful material exhibiting both MR and EPIR properties, that is non-toxic and easily integrable with electrode and other materials for device applications, without suffering from drawbacks, such as sensitivity to surface environment. High purity ruthenate perovskites might satisfy that need if they, and their related compounds, were not so difficult to form by conventional ceramic processing routes using mixed starting oxide powders and solid state reactions. Thus, there has been a further need, until the present invention, for a production method for synthesizing chemically uniform ruthenates and doped ruthenates that exhibit both large MR and EPIR effects, wherein the method significantly reduces the processing time and results in excellent compositional uniformity, as verified by diffraction and magnetic measurements.